July 6, 2007

That's the name of the new book by Steven E. Landsburg and the name of his 1996 article arguing "that H.I.V. would spread less quickly if relatively chaste people each took on a few more sexual partners":

At a given bar on a given night, he wrote, these disease-free singles would then make the pool of sexually active adults safer. The article was based largely on an academic paper by another economist, Michael Kremer, theorizing that the spread of AIDS could be slowed in England if everybody with fewer than about 2.25 partners got around a bit more

The link is to a book review by David Leonhardt, who doesn't think much of what he calls the the economists' "imperialist movement" -- intruding their analysis into all sorts of human affairs:

[The book is] short on the nuance that comes from real human stories. Landsburg’s characters tend toward the hypothetical... and his arguments, as he puts it at one point, can sound like “idle Sunday dorm-room chitchat.”

This problem plagues many of the new economic imperialists: like the overly chaste singles who are supposedly contributing to the H.I.V. epidemic, they don’t get out enough. They are asking good questions about epidemiology and psychology, but they are not spending much time with epidemiologists and psychologists, let alone with the people who are the subjects of their academic research.

IN THE COMMENTS: One of the coolest things about blogging: the author of the book appears and defends himself (in a thread that hilariously begins with the comment "He was great on Barney Miller").

The real problem here is that he is fundamentally wrong. In his scenario there are a fixed number of infected people. By increasing the number of uninfected, he increases the chance that any given uninfected person will hook up with an uninfected person. So far, so good. However, it also increased that chances of any given infected person will hook up with an uninfected person.

Since the number of infected to uninfected hookups increase, so do the number of transmissions.

The rate of infection goes down per sex act, but the rate of infection goes up per capita.

Otherwise, I get too damned angry whenever I see this so-called "new idea" referenced.

[The book is] short on the nuance that comes from real human stories. Landsburg’s characters tend toward the hypothetical... No sh*t, Sherlock.

As a real human being approaching the 20th anniversary of living with H.I.V., and nearly dying from it, as well as the first anniversary of drug treatment failure, I have a message for Steven Landsburg:

It is safer only for a day, for a person who 1. is not currently infected, and 2. walks into a room full of some people who are infected and some who are not.

In the long run, it is not at all safer, and will lead to a larger number of people being infected.

To see how this is true, consider what the case would be if HIV were transmitted by any other method. Would you be safer if you were in a room, for example, with one person with a cold, or fifty people, one of whom had a cold? Obviously with fifty because you would have some 'protection' in that most of the time they would be sheilding you from the person with the cold. But, the chances are that instead of possibly infecting one person, the person with the cold will infect several.

Yea, but Mr. Landsburg would argue that if the sex is normal, only one person would be exposed per infected person on each day.

That is true, but does he really want to limit this to a one day thing? Mehthinks not. And if not, then he is in effect saying that the infected person is like the person with the cold, since he will be spraying his infection into many people, not just a few.

Truthfully, I think that Mr. Landsburg wants to promote a world in which anyone can have sex with anyone else, for reasons known only to him.

I always enjoyed Landsburg's columns and books. I don't think he's trying to teach morality or sex or politics. He's trying to teach economics. He uses things like statistics and most importantly, the concept that incentives matter to analyze an issue and usually come to counterintuitive and frankly, bizarre conclusions. I guess I don't get the sense that he actually believes the conclusions but I take it as a challenge to find the flaw in the logic.

I agree, but this particular chapter was written not teach economics but to sell books with complete disregard for the realities of life to boot, of which an economist should be marginally cognizant at the least, I think.

Reader Iam:No need for me to add anything.

Alas, you know me too well. Be that as it may, I am about to get up on my high horse and depart for greener fields, so I expect you may have reason to add things later. Or have you already mastered typing on your iPhone?

The goal is not to minimize STDs. To do that, we'd outlaw sex entirely. Nobody has that goal. Instead, the goal is to maximize the excess of benefits (from consensual sex) over costs (from STDs, etc.). And it is economists, not epidemiologists, who are experts in trading off costs against benefits.

The first chapter of my book argues that more sex by chaste people would improve the world (in the excess-of-benefits-over-costs sense of "improve"). The argument relies only on standard principles that are taught in every economics course. To deny the conclusion, you'd need to find a faulty step in the argument. And since the argument is quite independent of epidemiology, the epidemiologist's expertise is quite irrelevant.

I think I see the author's point - that spreading the infection out over a significantly less promiscuous population will statistically slow down its spread.

To some extent, this is in line with some study or something I saw a week or so ago about how level of prostitution in a population has a bearing on the level of HIV (IR, et al. - this presumably involves non-first world populations, where much of the transmission goes through the prostitute population).

Ignorance is Bliss said... The real problem here is that he is fundamentally wrong.

I'm an economist. well actually one degree is economics but the Masters is in Operations Research (applied math)

Yes, some soldiers can read and write :)

anyway, Bliss is correct, hence I'm not calling him ignorant here.

Though introducing disease free folks into the equation makes it safer for the folks with loose morals, it overall increases the number of HIV inflections. Let's take the absurd example.

1. say you have 1 infected male in a room with 2 infected women. He has sex with 1. the net increase in HIV is 0.

2. now add 2 disease free women to the mix. now there is a 50% likelihood (assuming high transfer) that instead of 2 female HIV carriers, you now have 3. Have enough sex, and you have 4 infected women, not 2.

And since the argument is quite independent of epidemiology, the epidemiologist's expertise is quite irrelevant.

I agree, the argument is quite independent of epidemiology, and as an abstract argument appears quite sound. (You are, after all a professional economist and I am not.) It seems to me, however, that the real life application of such an argument is decidedly not "quite independent of epidemiology" and that the "epidemiologist's expertise" you refer to will quickly become quite relevant.

The title, and the argument, will help you sell a lot of books, I am sure. I do hope that none of those chaste persons in at-risk populations who take your suggestion to heart end up contracting HIV in consequence.

The goal is not to minimize STDs. To do that, we'd outlaw sex entirely. Nobody has that goal. Instead, the goal is to maximize the excess of benefits (from consensual sex) over costs (from STDs, etc.). And it is economists, not epidemiologists, who are experts in trading off costs against benefits.

The first chapter of my book argues that more sex by chaste people would improve the world (in the excess-of-benefits-over-costs sense of "improve").

I think that Steve is arguing that,

1. ignore the epidemiologists, yes, the number of HIV transmissions would increase, but that is on the "cost" side.

2. Lots more people would be having "safe sex" that is on the "benefits" side.. so looking at a cost/benefit ratio, the world would be better even as the number of HIV positives increases.

Though introducing disease free folks into the equation makes it safer for the folks with loose morals, it overall increases the number of HIV inflections.

Yes, it can, and probably will.

There are other factors, as well. Race is a huge factor in the incidence of new infections. As we don't "do race" well in public discussions, it is usually a mistake to bring it up.

Perhaps the economists here can explain what I've misunderstood:

Assuming that the dating pool is a genuine melting pot and all potential pairings are equally plausible returns results that are inconsistent with everyday reality.

(BTW, not to pick nits (which means I am about to), but it seems to me that safe sex is only possible with oneself (and some have a tough time with that). The real choices one has to make are no sex, sex and safer sex. Accidents do happen. Condoms do break.)

(BTW, not to pick nits (which means I am about to), but it seems to me that safe sex is only possible with oneself (and some have a tough time with that). The real choices one has to make are no sex, sex and safer sex. Accidents do happen. Condoms do break.)

Well from the gyn perspective, this is just nuts. Women are considered high risk for cervical cancer when the number of partners that they have had in a lifetime is 5. Increasing the number of partners and decreasing the age of first intercourse significantly increases the lifetime chance of acquiring an STD.....

I try to tell my patients the fewer the partners in a lifetime the better....

The phrase is "more sex is safer sex," not "more sex is more sex and assuming sex is generally good, there are more total benefits weighed against the costs of disease, depending on just how relatively bad you think it is to have HIV." I still don't see how more sex is safer sex. But it is a catchy phrase. I'll give you that. It made me blog about it.

And someone who is now chaste probably suffers less (if at all) from the lack of sex with a partner, so the idea that this person gains from plunging into a life of freer sex is dubious. The people who are already having a lot of sex are probably the one who want it the most and get the most good from it.

I'm not getting the "more sex is safer sex" thing at all. Setting aside pregnancy--which is pretty damn dangerous--it does seem like the assumption is that all possible pairings are likely.

If, by some fiat, the relatively chaste were to become a little less chaste, I imagine the most likely pairings would be among those who had previously been chaste. If that sentence is even remotely comprehensible.

In other words, monogamous and low-partner people by-and-large might, in fact, be put off (or intimidated) by sex with more experienced people. And vice-versa, the promiscuous might find the inexperienced too boring.

But even if that weren't the case, I'm not seeing how the benefit goes up for society--even disallowing the hugely negative consequences that would occur.

Let's say the rate of STD transmission was 5% in the average pool of sex-havers. So in 100 encounters 5 additional people would contract the disease. If people with a lower probability of having an STD are injected into the pool so the overall rate dropped to say 3% it would take 166.67 sexual encounters for 5 additional people to get the disease. If only 150 acts of sex were had then there was indeed more sex (150 rather than 100) and safer sex (4.5 diseases passed rather than 5.

The concept of 0.67 of a sexual encounter is left as an exercise for the reader.

The first fundamental flaw is that you assume that the value of the promiscuous sex at some point exceeds the cost of the transmission of aids. You provide no economic basis for this assumption. It could be that the only economically sound choice would be to have no promiscuous sex and no transmission of aids. In this case, all of the current promiscuous sex would be a bad choice from an economic perspective. The fact that people are currently making those mistakes does not imply that more would be better.

The second fundamental flaw in your reasoning is your assumption that each act of sex has the same value, and that each person values their health the same amount. If that were true, then your argument might be correct. If the people who are currently not promiscuous would not enjoy the additional sex very much, then the total enjoyment might increase at a lower rate than the increase in transmissions. In that case, the world would be worse off.

I was watching an infomercial late last night (I have no life and get no sleep) that may have some relevance here. It was about a product that enhances a certain part of the male body....at any rate, the host was interviewing these really hot women, all of whom said that they preferred macro-economists over micro-economists.

Ann Althouse said... mindsteps: "while having sex one is unable to contribute to the blogosphere."

That is so not true. You lack imagination.

Though doing both is certainly anatomically possible and likely has been done, is is easy to argue that not only can one not devote ones full attention to both at the same time, it is arguably pretty rude to the non-typing partner. :)

The phrase is "more sex is safer sex," not "more sex is more sex and assuming sex is generally good, there are more total benefits weighed against the costs of disease, depending on just how relatively bad you think it is to have HIV." I still don't see how more sex is safer sex. But it is a catchy phrase. I'll give you that. It made me blog about it.

I hope you will read the book, or at least the chapter; if you do, you'll find that there are several related points, and I am careful to distinguish among them.

The first point is that under quite reasonable assumptions drawn from standard epidemiological models, more sex probably IS safer sex; i.e. it actually reduces the spread of STDs. That conclusion relies on some modeling assumptions, but the assumptions are nowhere near as naive or silly as many of the commentators in this thread seem to assume.

The second point is that REGARDLESS of your modeling assumptions, and REGARDLESS of whether more sex actually reduces STDs, more sex (by the relatively chaste) MUST increase the excess of benefits over costs, not as a matter of idle speculation but as the conclusion of a careful logical argument.

And the third point is that following logical arguments is both illuminating and fun. Far more illuminating and far more fun than constructing and attacking straw men, in fact.

under quite reasonable assumptions drawn from standard epidemiological models, more sex probably IS safer sex; i.e. it actually reduces the spread of STDs.

1. I have had some fun with the topic, however I am interested in the analysis. Ok, while it may not be possible to lay it all out in a comment....what are some of the reasonable assumptions of the model

2. You wrote "more sex (by the relatively chaste) MUST increase the excess of benefits over costs, not as a matter of idle speculation but as the conclusion of a careful logical argument."

I know I'm not smart and all, but this really strikes me as making as much sense as that stupid idea that (according to media reports I've read) exits in parts of Africa: that AIDS can be cured by having sex with a virgin.

The second point is that REGARDLESS of your modeling assumptions, and REGARDLESS of whether more sex actually reduces STDs, more sex (by the relatively chaste) MUST increase the excess of benefits over costs, not as a matter of idle speculation but as the conclusion of a careful logical argument.

Wrong. This is only true if you assume there is already an excess of benefits over costs. You have not provided any basis for this assumption. Also it assumes that the addition sex provides as much additional value as the existing promiscuous sex.

Note that I'm not claiming to prove that your conclusion is wrong, only that you have not proven it.

1. I have had some fun with the topic, however I am interested in the analysis. Ok, while it may not be possible to lay it all out in a comment....what are some of the reasonable assumptions of the model

Answer: See the paper by Michael Kremer referenced by Adrian elsewhere in these comments.

2. You wrote "more sex (by the relatively chaste) MUST increase the excess of benefits over costs, not as a matter of idle speculation but as the conclusion of a careful logical argument."

What is the logical argument and does it have empirical support.

The following is a general economic principle: Activities that impose costs that are not felt by the decisionmaker are oversupplied and activities that impose benefits that are not felt by the decisionmaker are undersupplied. This is not an *assumption*; it is a *conclusion* drawn from quite fundamental principles; you can find the argument in any good intermediate level microeconomics textbook (including mine).

In the book, I spell out two quite separate ways in which sexual activity by the relatively chaste confers benefits not felt by relevant decisionmakers, from which the conclusion follows.

I apologize for not being able to spell this out in more detail here, but of course if it could all be said in a few paragraphs, I wouldn't have written a book.

(I'll note too that the book contains dozens of additional surprising consequences of the same principle.)

Rather than risk overstaying my welcome in someone else's blog, I'll bow out here---but of course if you want to know more---and to see the argument in detail---I hope you'll read the book!

I don't see why this assertion is susceptible to logical analysis -- careful or not. Isn't it just any article of faith that sex with a partner produces happiness?

The assumption is that on average, sex produces the expected amount of happiness. Sometimes much more than expected, sometimes much less than expected, but not systematically one or the other. The justification for that assumption is that if people are systematically wrong in a predictable direction, they'll eventually learn and correct for that.

That turns out to be all you need to get the analysis going (and if you won't grant me this much of a start, do be aware that the same assumption underlies 90% of economic analysis, and, stereotypes to the contrary, economic analysis does yield lots and lots of accurate predictions).

Now you can reason as follows (this is highly stylized, but it gives the flavor): You take a new partner when the expected benefits to you exceed the expected costs and not otherwise. Sometimes that will cause you to turn away partners even when the total expected benefits (to you and other members of society) do exceed the expected costs (because you count only some of the benefits, not all of them). And because, on average, expected benefits equal actual benefits, it turns out that actual benefits-minus-costs would be higher if you took additional partners.

(The ways in which your taking an additional partner benefits your neighbors are, incidentally, more substantial---and more subtle---than those mentioned in the New York Times review.)

I thought safe(r) sex meant using a condom. Why would an educated person suggest unprotected casual sex? AIDS sucks without the benefit of the suction. Even with a condom in place, sex is usually much better than using your hand. Or so I'm told...

The assumption is that on average, sex produces the expected amount of happiness. Sometimes much more than expected, sometimes much less than expected, but not systematically one or the other. The justification for that assumption is that if people are systematically wrong in a predictable direction, they'll eventually learn and correct for that.

I won't grant you that much, because that assumption is not justified in this case. The more chaste people are behaving exactly as would be predicted for people who do not derive as much value from promiscuous sex. Therefore we cannot assume that the benefit to them will outweigh the increased risk of transmission.

Oh, and thank you for stopping by and answering questions. Even if you haven't convinced me you are correct, your responses are appreciated.

And the third point is that following logical arguments is both illuminating and fun

That should have been the first point. I majored in Economics at Regis and I got to the point where I could no longer enjoy studying it PRECISELY because of poo like this. After reflecting on all the crap I got A's on, there's no way I can pay attention to an economics professor without thinking, "from whose bum did you pull those figures?"

Contrary to Landsburg's contention, epidemiology does trump economics in this instance. The more people you have sex with, the better your chance of having sex with someone with an STD. And the more often you have a sexual encounter with someone with an STD, the more likely you are of catching an STD.

True, the larger the pool of available non-infected partners, the lower the chance of an encounter with somebody with an STD. However, more sexual encounters for you means more sexual encounters for everybody else. Including those with incurable STDs such as drug resistent syphilis and AIDS.

In the long run you wind up with more infected people then you had before. Which means more money expended on treatment, a greater loss of productivity as the disease progresses. In short, a greater loss to the economy than otherwise.

True. In this particular case, it appears that the benefits of publicizing a deftly phrased but ultimately sterile economic model in order to maximize one's personal profit outweighs the cost of recognizing the unavoidable destruction of human lives were it to be followed in the real world. As is now obvious, all Lansdsburg chooses to discuss are the costs & benefits of the sex act, as if there are no other consequences of sex save satisfaction (on average, as he says).

The assumption is that on average, sex produces the expected amount of happiness.

Ah. For anyone whose reasons for not having more partners are a personal choice rather than an externally imposed one, partners beyond the desired number (say, one, for a monogamous person) would be expected to produce unhappiness.

So, in order for your conclusion to be true, you have to assume human nature is such that the relatively chaste are significantly that way due to external considerations.

blake: Ah. For anyone whose reasons for not having more partners are a personal choice rather than an externally imposed one, partners beyond the desired number (say, one, for a monogamous person) would be expected to produce unhappiness.

Yes. This is PRECISELY THE POINT. There is a disconnect between what's good for the individual and what's good for his/her neighbors. Those who are staying home on Saturday nights are presumably staying home because they've weighed costs and benefits and figured out that they OUGHT to be staying home on Saturday nights. The problem (socially) is that in doing so they've weighed all of the costs but only some of the benefits.

The analogy is with the owner of a polluting factory. Nobody claims that he's making a mistake by polluting. Instead, the claim is that out of selfishness, he pollutes more than is optimal for the world as a whole. And nobody claims the chaste are making a mistake in their chastity. Instead, the claim is that out of sefishness, they behave more chastely than is optimal for the world as a whole.

Again, the reason for this is that more sex by the chaste has MULTIPLE external benefits (i.e. benefits for others), which you can read about in the book (or for that matter in Kremer's article). Silly toy models like the one, say, in mythusmage's post above quite fail to account for these very real benefits.

In a large and diverse population, different people will assess the benefits and costs of promiscuous sex differently. Some see the benefits outweighing the costs, so they have sex. Others see the costs outweighing the benefits, so they don't. Some are on the borderline, where they choose to not have sex because the risks to themselves ever so slightly outweigh the benifits to themselves. If that borderline person were to also take into account the benefit that their having sex reduces someone else's chance of getting an SDT, that should change their assessment, the total benefits would outweigh the total costs, and they should have sex.

The problem with this is that, while you say they should look at the total benefit to the whole world, you don't advise them to look at the total cost to the world. While the most severe cost of AIDs is paid by the person who has it, there are significant costs to society too. If the borderline person takes these costs into consideration too than they may well still choose to not have sex.

In fact, there may well be some people for whom the benefits to themselves slightly outweigh the costs to themselves, who should stop having sex if they take into account both the benefits and costs to the rest of society.

As a layperson, I find the discipline of economics to be powerful, perplexing, and sometimes counter-intuitive. Interestingly enough, Barry Schwartz, a professor of psychology at Swarthmore College has an interesting op-ed in the 7-2-07 NYT on the proposal to pay students for academic achievement and the idea that sometimes economists base their assertions on mistaken assumptions. Here is a portion of the op-ed.

"The assumption that underlies the project is simple: people respond to incentives. If you want people to do something, you have to make it worth their while. This assumption drives virtually all of economic theory.

Sure, there are already many rewards in learning: gaining understanding (of yourself and others), having mysterious or unfamiliar aspects of the world opened up to you, demonstrating mastery, satisfying curiosity, inhabiting imaginary worlds created by others, and so on. Learning is also the route to more prosaic rewards, like getting into good colleges and getting good jobs. But these rewards are not doing the job. If they were, children would be doing better in school.

The logic of the plan reveals a second assumption that economists make: the more motives the better. Give people two reasons to do something, the thinking goes, and they will be more likely to do it, and they’ll do it better, than if they have only one. Providing some cash won’t disturb the other rewards of learning, rewards that are intrinsic to the process itself. They will only provide a little boost. Mr. Fryer’s reward scheme is intended to add incentives to the ones that already exist.

Unfortunately, these assumptions that economists make about human motivation, though intuitive and straightforward, are false. In particular, the idea that adding motives always helps is false. There are circumstances in which adding an incentive competes with other motives and diminishes their impact. Psychologists have known this for more than 30 years."

The entire piece, entitled "Money for Nothing" (I notice he did not cite Mark Knopfler) can be found here:

In a large and diverse population, different people will assess the benefits and costs of promiscuous sex differently. Some see the benefits outweighing the costs, so they have sex. Others see the costs outweighing the benefits, so they don't. Some are on the borderline, where they choose to not have sex because the risks to themselves ever so slightly outweigh the benifits to themselves. If that borderline person were to also take into account the benefit that their having sex reduces someone else's chance of getting an SDT, that should change their assessment, the total benefits would outweigh the total costs, and they should have sex.

This is correct, where it's important to understand (as I think you do understand) that the "should" in the last line refers to what's good for society, not what's good for the individual decisionmaker.

As I pointed out in the earlier post, this is exactly analogous to the polluting factory owner who decides to dump another bucket of sludge because the benefits to him slightly outweigh the costs. Accounting for those costs, he should make a different decision.

Ignorance is Bliss goes on to:

The problem with this is that, while you say they should look at the total benefit to the whole world, you don't advise them to look at the total cost to the world. While the most severe cost of AIDs is paid by the person who has it, there are significant costs to society too. If the borderline person takes these costs into consideration too than they may well still choose to not have sex.

The problem here is that you have not identified significant costs that fall on others. Don't forget you're probably (see the Kremer paper) making it *harder* for other people to get AIDS---largely by putting yourself at risk of getting infected, dying, and taking the virus with you. That's a benefit to your neighbors, not a cost. And it's only one of the benefits. And Kremer's research suggests that it's a benefit that is empirically large.

There is a disconnect between what's good for the individual and what's good for his/her neighbors. Those who are staying home on Saturday nights are presumably staying home because they've weighed costs and benefits and figured out that they OUGHT to be staying home on Saturday nights. The problem (socially) is that in doing so they've weighed all of the costs but only some of the benefits.

Self-sacrifice like this in the name of a supposed greater public good sounds like a very bad idea. You continue to ignore the facts of real life, such the composition of the dating pool and the sub-sets therein, looking only at an imaginary "big picture" in order to confirm what you wanted to find. You persist in eliding the fact that the cost/benefit ratio is dramatically different for sub-sets within that the pool you yourself built (some of which you appear to recognize, others you ignore at your convenience, I suspect).

You persist in eliding the fact that the cost/benefit ratio is dramatically different for sub-sets within that the pool you yourself built

You very clearly do not understand the argument. All that matters is that for some individuals at some times in some circumstances, the decision not to take an additional partner is a close call. In those cases, an accounting for external benefits would reverse the decision. So some decisions go the "wrong" way. Costs and benefits can differ across groups or individuals in any way you like without affecting that argument.

As for this:

In other words, the individual must subordinate the natural instinct for self-preservation for the greater good.

The assertion is that the world would be a better place (in a very precise sense) if a certain class of individuals had more sexual partners than is good for them. If you're jumping to the conclusion that those individuals are somehow therefore morally obligated to have more sex, then your morality is very different from mine.

If Mr. and Mrs. Smith have been married to each other for many years, have a good sex life and have sex with each other, then that improves the benefit of life for each of them.

It is hard to see how their going to a bar, which leads to Mr. Smith having sex that night with Ms. Jones, and Mrs. Smith having sex that nigh with Mr. Chapman, improves their lives at all. It might make life better for Ms. Jones and Mr. Chapman, but then if they desire sex so much, why don't they hook up with each other and leave the Smiths alone?

Further, you also neglect the other obvious argument against this kind of set up. Suppose that Mrs. Smith becomes pregnant and paternity tests, ordered by Mr. Chapman show that the kid is his, not Mr. Smith's. Then you consign that child to grow up in a more difficult situation (and there is no doubt that children whose parents are either divorced or never were married do have more problems later in life, there are many studies which show that to be the case). Leaving aside the negative fallout from the matter of the Chapman/Smith child in the lives of Mr. and Mrs. Smith and Mr. Chapman (and presumably at some point Mrs. Chapman), and also assuming that all parties and their lawyers are honorable in their intentions regarding the child, it is hard to argue that the benefit to societal happiness from such a sexual encounter is such that it outweighs the larger amount of misery and confusion which the child will feel from having to shuttle back and forth instead of (as would have happened if Mrs. Smith conceived with her husband) the lack of confusion and sure sense of identity that the child gains from growing up 100% of the time in the Smith household.

I should add, Steven, that I am positive that you are professional enough that your model works within the narrow confines of your construct. It undoubtedly can be used to explain any number of activities and outcomes. You chose to use this subject to illustrate your model.

You obviously made an excellent choice and your book sales will be much higher as a result.

Whether or not, like most castles built on air, it genuinely describes the real consequences and potential outcomes of decisions made in real life is, I think, open to debate, as many of those potential outcomes and consequences are deemed irrelevant to the model. As you have already pointed out, there are good reason in terms of what specifically is being modelled, but out here in the real world, human beings rarely have the luxury of operating in a pristine vacuum.

The assumption that sex increases happiness, is simplified to the matter of the act itself. In terms of its cost, you have focused only on STD's and ignored the other big bugaboo that sex can cause-- children created by sexual encounters with persons other than who their parents would prefer they be raised by.

The problem here is that you have not identified significant costs that fall on others. Don't forget you're probably (see the Kremer paper) making it *harder* for other people to get AIDS---largely by putting yourself at risk of getting infected, dying, and taking the virus with you. That's a benefit to your neighbors, not a cost. And it's only one of the benefits. And Kremer's research suggests that it's a benefit that is empirically large.

But Kremer's analysis is based on epidemiology. I am perfectly willing to believe that based on the right combination of population behavior and transmission rates adding in extra people who will have a small amount of extra sex will reduce the overall infection rate.

But you claim that you can prove increased excess benefit based on economics alone. Based on economics alone we would expect the number of infections to go up. If there are extra infections there are extra costs to society in addition to the costs to the infected individual, so the marginal case should consider those costs in addition to considering the extra benefits to society.

Geez, nothing like injecting a little sex into an economic discussion to send it all to hell. I highly recommend this book and Landsburg's other writings. As mentioned before he applies basic economic principles to situations we are all familiar with and comes to sometimes very counterintuitive conclusions. They provoke us to re-examine our own views of the situation and if we disagree with his conclusions then we are challenged to find the logical flaw. I've found it very useful in understanding economics. I think applying economics to situations like these spices up a very dry subject (economics not sex).

As far as his titling his book to sell more copies, I'm as shocked as you are.

Those who are staying home on Saturday nights are presumably staying home because they've weighed costs and benefits and figured out that they OUGHT to be staying home on Saturday nights.

What about the fact that studies show that people in long-term monogamous relationships (i.e. the happily married) have more and better sex and are more satisfied with their sex lives than those who engage in more casual sexual encounters? Even if these are not the people we're supposing are going to be adding another partner or two, by taking relatively chaste people and expanding their partner pool, I would suppose you also lower their chances of finding that happy monogamous relationship (studies back that up, iirc). So even without factoring in disease transmission, I would contend that overall levels of happiness or even sexual satisfaction would not go up.

And as to the OP article: See, "more sex is safer sex" only applies if the "unsafe" population is a fixed number, which is not the case. On a long enough timescale, the "unsafe" population will be the only population.

Of course, you can mask this growth for a while by increasing the total population more quickly than the unsafe population grows, but eventually your total population includes every human being on the planet and you can't grow any more...

A couple thoughts. First off, I’m surprised that no one has yet mentioned the Freakonomics authors’ piece in the New York Times Magazine on “The Economy of Desire,” which looked at how AIDS may effectively “raise” the price of sex for some.

Though I was an admittedly lazy econ student, I tend to think applications like the above, which use economics to explain motivations for choice rather than proposing changes in sexual behavior, are more helpful uses of economics to assess human behavior.

Furthermore, it seems to me that the differences between men’s and women’s sexual experiences/motivations/risks have been inadequately distinguished in the conversation so far. Without have read the book in question, I obviously can’t say whether he addresses this or not, but it is extremely problematic to equate the two, particularly in the case of STDs. As Steven Rhoads reports in his book Taking Sex Differences Seriously, “For one act of intercourse, for example, a woman is more than eight times as likely to get HIV and about four times as likely to get gonorrhea as her male partner.” Differences like that simply can’t be ignored if one wishes to do responsible analysis, economic or otherwise.